One of the first operations in the manufacture of fabrics in a textile mill is a process known as "wraping." Wraping involves unwinding threads from their individual spindles and collecting them onto a large drum by rewinding them in side by side relationship onto the drum. The apparatus that performs this function is known as a "wraper."
The wraping operation should be performed as quickly as possible, with all threads wrapped at even, or substantially even tension, and without any interruptions, for example due to thread breakage ("broken end"). In some cases, variations of 10% in tension from thread to thread are considered unacceptable. In practice, however, fluctuations in tension occur and measures must be taken to keep the fluctuations within tolerance, for example, adjustment of the thread tensioning device associated with the wraper. Additionally, thread breakages must be detected as soon as possible so that the wraper can be stopped before the broken end of the thread is lost on the drum. Failure to meet these requirements can result in quality control problems and costly down time.
In the prior art, mechanical devices are commonly employed to detect thread breakage. The most commonly employed device comprises a cantilevered arm whose free end urges against the thread. The arm is coupled to an electrical contact that opens (or breaks) when the thread breaks. A significant disadvantage of this device is that it is incapable of providing indications of tension fluctuations that occur during operation of the wraper. Another disadvantage is the unreliability of a mechanical contact due to corrosion, dirt and thread filaments.
U.S. Pat. No. 4,883,531 describes a thread friction measurement device that employs u pair of strain gauges coupled to a pair of guide rollers over which the thread rides. It is said that the strain gauges provide electrical outputs corresponding to the tension in the thread before and after turning around the roller guides. However, devices of this type have numerous drawbacks which limit their practical use as a thread tension and breakage sensor. First, the imposition of the guide rollers in the thread path adds friction and hence increases the thread tension beyond that set by the tensioning device. The friction imposed by the rollers can be great enough to itself cause the thread to break, especially if the thread is of thin gauge. Second, the thread weaves an elaborate path through the device and hence the thread is difficult to reweave through the device after a breakage has occurred. Third, plural sensors, in this case two strain gauges, are required to obtain a measurement of thread tension and thus these types of devices are relatively expensive to manufacture. Fourth, due to the complexity of the device, it is unlikely that it can reliably provide the sensitivity required for tension detection, or that it can reliably react to a broken thread with the requisite speed. Fifth, strain gauges are static devices that provide outputs irrespective of their loading status, and circuitry such as null and span adjustments must be provided for calibration. Drift can occur over short periods of time, and/or as a result of temperature and/or humidity changes, thus causing erroneous outputs.
It is therefore desirable to provide a thread sensor for use with a tensioning device associated with the wraper which is simple, reliable, relatively inexpensive to manufacture, and which does not substantially alter the thread path and does no introduce appreciable friction and hence tension to the thread. It is also desirable that the thread sensor be capable of sensing as little as 10% fluctuation in thread tension and react very quickly to thread breakage. The present invention achieves these goals.